Fonz, Jose Miguel Mora; (2007) The pre-nucleation of zeolites: a theoretical approach. Doctoral thesis , UCL (University College London).

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Abstract

The reactions and clusters involved in the first stage of nucleation of high silica zeolites are studied using computational methods. A method is developed which gives accurate energetics compared with experiment. Characteristic thermodynamical properties (enthalpy, entropy and Gibbs free energy) for the reactions of silicates up to 12 silicon atoms are presented. The silicate geometry and energy were calculated with a standard density functional method (DVT): BLYP/DNP//BLYP/DNP. A DFT simulated annealing is performed before the geometry optimisation, to avoid local minima. Solvation is modelled with a continuum dielectric method (COSMO), and in some cases with explicit water molecules the thermodynamical properties (including Zero Point Energy Correction) are calculated with standard statistical mechanical methods. High accuracy in deprotonation energies could be obtained with the model employed. The results reveal the crucial role of high pH in creating anionic silicate species, which can condense linearly, while solvation offers a stable environment for those anions. The important cyclic condensations, which produce the ring structures present in every zeolite crystal, are driven by a large positive change in the entropy. pH and temperature in some cases allow selective condensations. However, uniquely, 4-ring species are found to be very stable under most of the conditions considered (pH and temperature). The conditions for the specific cyclization of important ring structures including three, four, five and double-four rings is discussed. The interaction between silicate clusters and water molecules is investigated and an important silicate-silicate interaction is found which could be responsible for the driving force to form silicate aggregates, a crucial step in zeolite synthesis.

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